This is the commonest hoverfly in our garden at the moment - Rhingia campestris. Here it's on sweet rocket Hesperis matrionalis. When the insect is at rest its proboscis sticks out in front and looks quite menacing, but when it's actually feeding ..
..... the proboscis hinges downwards and the stretched soft tissue between it and the 'snout' that faces forward acts like a pump for sucking up nectar.
This individual has just withdrawn its proboscis from the flower, giving a better impression of how long it is. It's often assumed that this hoverfly is exclusively a nectar feeder but a study published back in 1989 by John Haslett at the Department of Zoology at Oxford University found substantial amounts of pollen in the guts of females, which need the protein that it contains for the production of their eggs. Male R. campestris were mainly nectar feeders, using the sugars as their energy source.
The books say that R. campestris breeds in dung, something of a contrast to the smell of sweet rocket that it's feeding on here - this is one of the most fragrant plants in the garden, with a strong carnation scent.